The present invention relates generally to optical networks that employ wavelength division multiplexing and, more particularly, to optical networks that use optical cross-connect switches in transit and hub nodes.
Many existing long distance telecommunication systems include spans of optical fiber that link digital switches in a network. Such systems, however, often operate at a single transmission wavelength and use time division multiplexing, which can restrict expansion of the system to handle larger volumes of voice or data communications.
Wavelength division multiplexing (WDM) provides a technique to accommodate increased traffic in existing long distance telecommunication networks. WDM takes advantage of the large bandwidth of optical fibers and sends multiple communications down a single optical fiber in separate wavelength channels. As a result, a WDM system can multiply the capacity of the system compared with the use of only a single wavelength.
Other prospects for expanding the capacity of an existing network provide significant obstacles. For instance, adding new optical fibers to handle more traffic is expensive and can complicate management of the existing network. Also, increasing the bit rate of a single wavelength system can cause transmission problems, such as polarization mode dispersion or self-phase modulation. WDM can avoid these problems while using the existing fiber infrastructure.
Upgrading an existing optical network to WDM, however, may present additional problems when the multiple links between nodes in the network are not uniform. For example, two links between three network nodes may have different numbers of optical fibers. FIG. 1 illustrates an optical network 100 having links such as 105, 110, and 115 interconnecting nodes such as nodes A, B, and C. Link 105 has 6 fibers (each line representing 2 fibers), link 110 has 4 fibers, and link 115 has 2 fibers. Coordinating these differences in fiber capacity presents challenges to the cross-connect switches within nodes A, B, and C.
Moreover, incorporating some links and nodes already operating with WDM into a larger WDM network presents similar problems when the different links are not uniform. For instance, if some WDM has been used between the links, the number of wavelength channels, the wavelengths themselves, or the transport standards may differ. These inequalities may exist due to the current lack of standardization in many areas of WDM communications. Consequently, optical cross-connect switches (OXCs) within the various network nodes must interface the incoming and outgoing optical fibers while maintaining compatibility with the varying optical standards used by the neighboring links.
In order to permit optical communication through an existing network like 100, nodes A-F are equipped with OXCs whose task is to switch the optical channels coming from N input fibers to N output fibers. In general, the possible OXC architectures can be divided into two main classes: fiber-routing cross-connect switches (FR-OXCs), wavelength-routing cross-connect switches (WR-OXCs), and combinations of them. FR-OXCs are also known as optical switches.
FIG. 2 illustrates a typical scheme of a WR-OXC. The WR-OXC performs routing channel by channel, thereby allowing channels from the same input fiber to be sent to different output fibers. WR-OXC 200 includes demultiplexers 205 and 210 for separating the signals traveling in received WDM combs via optical fibers 215 and 220. After the demultiplexing, optional 3R regenerators or transponders 225 and 230 can provide both signal regeneration and wavelength adaptation for each of the wavelengths entering WR-OXC switch 235. Transponders 225 and 230, if present, convert the wavelengths to a grid used particularly by switch 235 and are typically realized by electro-optic techniques. This wavelength conversion, if present, makes the large network opaque with respect to carrier wavelengths, although all-optical networks are envisioned for the future that perform all phases of transmission, amplification, and switching in the optical domain. After passing through switch 235, where any entering wavelength may be switched to any exiting path, the respective wavelengths can pass through optional output transponders 240 and 245, are combined in multiplexers 250 and 255, and exit via optical fibers 260 and 265. Optional output transponders 240 and 245 can convert the carrier wavelengths to the particular value required by the network downstream of WR-OXC 200.
In contrast to the WR-OXC, an FR-OXC has the task of switching entire optical WDM combs between input fibers and output fibers without demultiplexing the optical channels. Adding or dropping of entire WDM combs from or to another optical network entity via the FR-OXC is also possible. Like the WR-OXC, the FR-OXC is statically configured based on a routing table and is reconfigured when the traffic changes. No regeneration is present in the FR-OXC, since the individual WDM channels are not demultiplexed. It is evident that once the number of input and output optical fibers is fixed, a WR-OXC has more switching versatility than an FR-OXC. Due to this increased complexity, WR-OXCs cost more than FR-OXCs.
In a wide area network, such as 100, each node processes and switches a certain number of local channels and a certain number of passthrough channels. The local channels are generated or terminated at destinations affiliated with the node, while the passthrough channels are routed through the node to elsewhere in the network. If the number of nodes is more than a few dozen, the majority of network nodes generally has many more passthrough channels than local channels. These majority of nodes are called transit nodes. The number of local channels exceeds the number of passthrough channels in only a few nodes, which are called hub nodes.
Patents and publications have contemplated the combination of WR-OXCs and FR-OXCs into a tandem switch structure in certain circumstances. For instance, U.S. Pat. No. 5,457,556 discloses an OXC system having a space switch with first and second inlet ports and first and second outlet ports. The first inlet and first outlet ports receive, switch, and transmit entire WDM combs within the space switch. A wavelength switch has first inlet ports connected to the second outlet ports of the space switch via a demultiplexer and first outlet ports connected to the second inlet ports of the space switch via a multiplexer. The wavelength switch receives selected WDM combs from the space switch, switches individual channels between the combs, and sends the switched channels back to the space switch.
In the ""556 patent, system protection occurs at a fiber or multiplex section level. Because protection switching is accomplished without demultiplexing a WDM comb, all incoming and outgoing fibers enter and exit the tandem OXC via a space switch, i.e. an FR-OXC. Accordingly, each node in the network, which operates with fiber level protection, has at least an FR-OXC as the input and output for the links to other nodes.
U.S. Pat. No. 5,805,320 discloses an OXC having a combined FR-OXC and WR-OXC where the FR-OXC handles the bypass component for reserve optical transmission lines in a fiber-level protection scheme termed Type A. The WR-OXC is connected to the working optical transmission lines, and connection links interface the two switches together. For Type B and Type C protection schemes that operate at the channel level, the ""320 patent discloses a combination of WR-OXC switches for both the working and reserve optical transmission lines without the use of an FR-OXC.
Applicants have observed that the known arrangements for OXCs within an optical network suffer from excessive complexity for fulfilling the needs of both transit nodes and hub nodes. In particular, Applicants have recognized that in an opaque optical network having a channel-level protection scheme, existing configurations of OXCs contain an overabundance of wavelength converters or transponders that leads to unnecessary expense and complexity.
Applicants have discovered that an opaque optical network that includes transit nodes and hub nodes can be configured more efficiently and cheaply by incorporating tandem OXCs within transit nodes that use FR-OXCs to switch WDM combs passing through the node and WR-OXCs to switch traffic headed for local destinations, and by incorporating at least WR-OXCs within hub nodes to directly receive and switch local traffic. Applicants have further found that an opaque optical network that has a channel-level protection scheme can be configured more efficiently and cheaply by having at least one OXC node of a tandem FR-OXC and WR-OXC architecture.
In one aspect, an optical WDM network consistent with the present invention communicates WDM channels to local destinations via a plurality of nodes and interconnecting optical fibers, where the nodes include at least one transit node and at least one hub node. The transit node includes a transit switch having a fiber-routing portion (FR) and a wavelength-routing portion (WR), where the fiber-routing portion includes a first group of FR inputs coupled to first network optical fibers and a first group of FR outputs coupled to second network optical fibers. The wavelength-routing portion is positioned in a feedback path between a second group of FR outputs and a second group of FR inputs. The fiber-routing portion is capable of switching a WDM comb from any of the first and second FR inputs to any of the first and second FR outputs.
Quantities of the first and second groups of FR outputs of the transit switch are selected such that CFR1 greater than CWR1, where CFR1 is a switching capacity of the fiber-routing portion and CWR1 is a switching capacity of the wavelength-routing portion. The network also has a hub switch within the hub node that has at least a wavelength-routing portion that includes a first group of WR inputs and a first group of WR outputs. The first group of WR inputs is arranged to receive first orders of WDM channels directly from some of third network fibers, and the first group of WR outputs is arranged to transmit second orders of WDM channels to local destination fibers.
In another aspect, an optical WDM network includes a plurality of first optical fibers each carrying WDM channels from at least one upstream node to a transit node and a plurality of second optical fibers each carrying WDM channels from the transit node to at least one downstream node. The network has a transit switch located in the transit node that includes a first optical switch having a plurality of inlets and a plurality of outlets. A first subset of the first-switch inlets is coupled to the first fibers, and a first subset of the first-switch outlets is coupled to the second fibers.
The transit switch also has a second optical switch with a plurality of inlets and a plurality of outlets and a plurality of input transponders and a plurality of output transponders coupled respectively to the second-switch inlets and the second switch outlets. At least one demultiplexer is positioned between at least one of a second subset of the first-switch outlets and ones of the second-switch inlets, and at least one multiplexer is positioned between ones of the second-switch outlets and at least one of a second subset of the first-switch inlets.
The network of the second aspect further includes a plurality of third optical fibers each carrying WDM channels from at least one upstream node to a hub node, a plurality of fourth optical fibers each carrying WDM channels from the hub node to at least one local destination, and a hub switch located in the at least one hub node. The hub switch has at least one demultiplexer coupled to at least one of the third fibers, a plurality of input hub transponders each coupled to a respective output of the at least one demultiplexer, and a third optical switch.
The third switch in the network of the second aspect has a plurality of inlets coupled to the plurality of input hub transponders and a plurality of outlets, and includes a plurality of output hub transponders coupled to the plurality of third-switch outlets, and at least one multiplexer positioned between the plurality of output hub transponders and the fourth optical fibers.
In a third aspect, an optical WDM network consistent with the present invention has a plurality of nodes interconnected by optical fibers and a channel-level protection scheme. The network includes a plurality of first network optical fibers each carrying first WDM channels that have an affiliated first optical channel header with information about the channel-level protection, and a plurality of second network optical fibers each carrying second WDM channels that have an affiliated second optical channel header with information about the channel-level protection.
The network of the third aspect includes at least one optical cross-connect switch made of a fiber-routing portion (FR) and a wavelength-routing portion (WR). The fiber-routing portion includes a first group of FR inputs coupled to the first network optical fibers and a first group of FR outputs coupled to the second network optical fibers. The wavelength-routing portion is positioned in a feedback path between a second group of FR outputs and a second group of FR inputs of the fiber-routing portion. Quantities of the first and second groups of FR outputs are selected based on an expected capacity of the fiber-routing portion with respect to the wavelength-routing portion.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the invention as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages and purposes of the invention.